I've cleaned out all the air filter/air intake housings/hoses.
They were really dirty, and I recently replaced the air cleaner element, but that got very dirty again quickly.

My independant mechanic friend said that because my engine is high mileage (267K), it is "breathing" more deeply, due to natural wear, and I could cure the dirty air element problem by disconnecting the crankcase emission hose, and diverting it into an old plastic bottle.
I note my BX has no crankcase emission filter as such.

I've changed the engine oil/filter on a periodic service, and I would say that the sump is full of sludge.
I know there are cleaning additives to cure this, but am I correct in thinking they should only be used immediately prior to an oil change?

The engine smokes a bit when cold, but is ok when warm (just like any diesel I guess).

Apart from that, the engine is still running fine, although it will need an overhaul one day.
I have a spare 1.7 diesel lump which has done 110K, while my Meteor has 111K on the clock.

The best way to clean out the sump is a few frequent oil and filter changes,perhaps every 1000 miles, your old XUD lump will be happy on an economy oil such as morris duplex, which should come out at well under a tenner a gallon, three pounds for a filter, which you should change each time too, soon all the gunge will be ceared, you might even unstick a sticky piston ring and improve the smoking too.
Stewart

Good plot moving the engine breather. As your engine is a starship miler it is passing crank case fumes containing oil vapor onto the air filter element. I have re-routed mine even though the fumes are not a problem, it just dangles down the front of the block. If you put the breather tube into a plastic bottle, don't seal the bottle as the fumes need to escape.
If you are changing the oils at regular intervals, I doubt you have sludge in the sump and I wouldn't use a flushing additive as you could wash away carbon and other build ups around the piston rings and cause it to start burning oil

You could also flush the engine at oil change with oil. Drain the old oil, fill with three litres of fresh oil, run for ten minutes at idle and then drain, apply new oil filter and replace with fresh oil. This tactic works and is cheap enough.